Because it is illegal (and dangerous for the underground Church) to attend the Roman Catholic Underground Church in China, you are not obligated to attend Mass on Sundays or other holy days of obligation. However, if you choose to go to Mass, you can attend the government-established Chinese Catholic Patriotic church, or a state-approved Eastern Orthodox church.
The Code of Canon Law states:
Whenever necessity requires it or true spiritual advantage suggests it, and provided that danger of error or of indifferentism is avoided, the [Catholic] Christian faithful for whom it is physically or morally impossible to approach a Catholic minister are permitted to receive the sacraments of penance, Eucharist, and anointing of the sick from non-Catholic ministers in whose Churches these sacraments are valid. (CIC 844 §2)
In his Letter to the Church in China (May 27, 2007), Pope Benedict XVI writes:
Concerning bishops whose consecrations took place without the pontifical mandate yet respecting the Catholic rite of episcopal ordination, the resulting problems must always be resolved in the light of the principles of Catholic doctrine. Their ordination as I have already said (cf. section 8 above, paragraph 12) is illegitimate but valid, just as priestly ordinations conferred by them are valid, and sacraments administered by such bishops and priests are likewise valid. Therefore the faithful, taking this into account, where the eucharistic celebration and the other sacraments are concerned, must, within the limits of the possible, seek bishops and priests who are in communion with the pope: Nevertheless, where this cannot be achieved without grave inconvenience, they may, for the sake of their spiritual good, turn also to those who are not in communion with the pope.